Potentially disastrous consequences can occur when pipes used for water or gas distribution experience a material failure. These pipes are generally subject to product standards and performance requirements set forth in norms such as, for example, DIN (German Industrial Norm or “Deutsche Industrie Norm”) or norms defined by ISO (International Organization for Standardization, Geneva, Switzerland). Pipes made from polyethylene may also need to meet the so-called PE80 or PE100 ratings (PE stands for polyethylene), which include the ability to withstand a minimum hydrostatic strength of 8 MPa (PE80) or 10 MPa (PE100) at 20° C. for 50 years.
Use of polyethylene resins in pipe applications is not without its disadvantages. Polyethylene resins can have relatively poor long term hydrostatic strength (LTHS) at high temperatures, which can render these materials unsuitable for use in piping that may be exposed to higher temperatures, such as, domestic or industry pipe systems. Other materials often used in domestic pipe systems include polybutylene, polypropylene, and cross-linked polyethylene (“PEX”). Polybutylene can be a very expensive material, while polypropylene can have less hydrostatic resistance at higher temperatures. PEX is also not without its disadvantages. Crosslinking can generate significantly higher costs than in thermoplastic pipe extrusion without crosslinking, and crosslinking can be difficult to control to achieve the proper crosslinking levels. Finally, PEX pipes cannot be welded together to form a piping system. Industry pipe systems mostly use polyethylene and polypropylene; however, when the pipe systems are exposed to higher temperatures and/or higher pressure, the pipe systems may degrade and burst or crack due to lower hydrostatic strength. Additional materials that may be used in domestic and industry pipe systems can include polyethylenes of raised temperature resistance (“PE-RT”), which are a class of polyethylene materials for high temperature and high pressure applications. These polyethylene materials are classified as PE-RT type 0, PE-RT type I or PE-RT type II based on their temperature and pressure resistance. The higher the type number the better may be the temperature and pressure resistance. Thus, in some applications, where higher pressure ratings are required, the PE-RT resins do not work due to less hydrostatic resistance at lower temperatures.
Beside hydrostatic strength, slow crack growth resistance is also important property for pipe applications as slowly developed micro cracks can also cause pipe failure. Slow crack growth resistance may be measured by the Pennsylvania Notch Test, or PENT in short. In general, a minimum of 500 hours of PENT is desired for most pipe applications.
Another important aspect of using polyethylene in pipe applications is the material's processability. In general, polyethylene materials with a low molecular weight (high melt index) and a low melt viscosity are easier to process, especially, for small diameter domestic and industry pipes, where high line speed is preferred to increase production rate. However, these resins do not meet the hydrostatic strength requirements at both room and elevated temperatures, as well as slow crack growth resistance. In order to meet these requirements, a very high molecular weight (low MI) and a bimodal molecular weight distribution may often be required. As a consequence, the resulting materials can be hard to process, especially for small diameter domestic and industry pipe, where high line speed is required.
Accordingly, it is desired to provide polyethylene pipe resins that have improved hydrostatic strength at higher temperatures and high pressure, as well as excellent slow crack growth resistance and improved processability.